High strength steel sheet having excellent workability and method for manufacturing same

ABSTRACT

Provided is a steel sheet which can be used for automobile parts and the like, and relates to a steel sheet and a method for manufacturing same, the steel sheet having an excellent balance between strength and ductility, an excellent balance between strength and hole expansion properties, and an excellent yield ratio evaluation score. The steel sheet includes: by wt %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities; and as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures.

TECHNICAL FIELD

The present disclosure relates to a steel sheet that may be used forautomobile parts and the like, and to a steel sheet having high strengthcharacteristics and excellent workability and a method for manufacturingthe same.

BACKGROUND ART

In recent years, the automobile industry has been paying attention toways to reduce material weight in an effort to protect the globalenvironment and secure occupant safety. In order to meet theserequirements for safety and weight reduction, the use of a high strengthsteel sheet is rapidly increasing. In general, it is commonly known thatas the strength of the steel sheet increases, the workability of thesteel sheet is lowered. Therefore, in a steel sheet for automobileparts, a steel sheet having excellent workability represented byductility, a hole expansion ratio, and the like, while having highstrength characteristics is required.

Since transformation induced plasticity (TRIP) steel, usingtransformation-induced plasticity of retained austenite, has a complexmicrostructure consisting of ferrite, bainite, martensite, retainedaustenite, and the like, it is known as having a certain level or moreof workability, as well as high strength characteristics.

As a technique for further improving the workability of a steel sheet, amethod of utilizing tempered martensite is disclosed in Patent Documents1 and 2. Since tempered martensite made by tempering hard martensite issoftened martensite, there is a difference in strength between temperedmartensite and existing untempered martensite (fresh martensite).Therefore, when fresh martensite is suppressed and tempered martensiteis formed, the workability may increase.

However, by the techniques disclosed in Patent Documents 1 and 2, abalance (TS²*EL^(1/2)) of tensile strength and elongation does notsatisfy the range of 3.0*10⁶ to 6.2*10⁶ (Mpa²%^(1/2)), meaning that itis difficult to secure a steel sheet having both of excellent strengthand excellent ductility.

Meanwhile, as another technique for improving workability of a steelsheet, Patent Document 3 discloses a method for inducing generation ofbainite by means of adding boron (B). In the case of adding boron (B), aferrite-pearlite transformation is suppressed, while generation ofbainite is induced, whereby coexistence of strength and workability canbe achieved.

However, by the technique disclosed in Patent Document 3, a balance(B_(TE)) of tensile strength and elongation of 3.0*10⁶ to 6.2*10⁶(mpa²%^(1/2)), a balance (B_(TH)) of tensile strength and a holeexpansion ratio of 6.0*10⁶ to 11.5*10⁶ (mpa²%^(1/2)), and a yield ratioevaluation index (I_(YR)) of 0.15 to 0.42 cannot be secured at the sametime, thereby meaning it is difficult to secure a steel sheet having allof excellent strength, an excellent hole expansion ratio, excellentductility, and an excellent yield ratio.

That is, a demand for a steel sheet having all of an excellent balance(B_(TE)) of tensile strength and elongation, an excellent balance(B_(TH)) of tensile strength and a hole expansion ratio, and anexcellent yield ratio evaluation index (I_(YR)) is not satisfied.

RELATED ART DOCUMENT

-   (Patent Document 1) Korean Laid-Open Patent Publication No.    10-2006-0118602-   (Patent Document 2) Japanese Laid-Open Patent Publication No.    2009-019258-   (Patent Document 3) Japanese Laid-Open Patent Publication No.    2016-216808

DISCLOSURE Technical Problem

According to an aspect of the present disclosure, a steel sheet havingall of an excellent balance of tensile strength and elongation, anexcellent balance of tensile strength and a hole expansion ratio, and anexcellent yield ratio evaluation index and a method manufacturing thesame can be provided, by optimizing a composition and microstructures ofthe steel sheet.

An object of the present disclosure is not limited to the abovementionedcontents. Additional objects of the present disclosure are described inthe overall content of the specification, and those of ordinary skill inthe art to which the present disclosure pertains will have no difficultyin understanding the additional objects of the present disclosure fromthe contents described in the specification of the present disclosure.

Technical Solution

In an aspect of the present disclosure, a high strength steel sheethaving excellent workability may comprise: by wt %, C: 0.1 to 0.25%, Si:0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S:0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe,and unavoidable impurities; and as microstructures, bainite, temperedmartensite, fresh martensite, retained austenite and unavoidablestructures, wherein the high strength steel sheet may satisfy thefollowing relational expression 1:

0.03≤[B] _(FM) /[B] _(TM)≤0.55  [Relational Expression 1]

-   -   where [B]_(FM) is a content (wt %) of boron (B) contained in the        fresh martensite, and [B]_(TM) is a content (wt %) of boron (B)        contained in the tempered martensite.

The steel sheet may further comprise, by wt %, one or more of thefollowing (1) to (8):

-   -   (1) one or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to        0.5%;    -   (2) one or more of Cr: 0 to 3.0% and Mo: 0 to 3.0%;    -   (3) one or more of Cu: 0 to 4.0% and Ni: 0 to 4.0%    -   (4) one or more of Ca: 0 to 0.05%, REM: 0 to 0.05% excluding Y,        and Mg: 0 to 0.05%;    -   (5) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%;    -   (6) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%;    -   (7) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%; and    -   (8) Co: 0 to 1.5%.

The microstructure of the steel sheets may include, by volume fraction,10 to 30% of bainite, 50 to 70% of tempered martensite, 10 to 30% offresh martensite, 2 to 10% of retained austenite, and 5% or less(including 0%) of ferrite.

In the steel sheet, a balance (B_(TE)) of tensile strength andelongation expressed by the following relational expression 2 satisfies3.0*10⁶ to 6.2*10⁶ (MPa²%^(1/2)), a balance (B_(TH)) of tensile strengthand a hole expansion ratio expressed by the following relational 3expression satisfies 6.0*10⁶ to 11.5*10⁶ (mpa²%^(1/2)), and a yieldratio evaluation index (I_(YR)) expressed by the following relationalexpression 4 satisfies 0.15 to 0.42:

B _(TE)=[Tensile Strength (TS, MPa)]²*[Elongation (El,%)]^(1/2);  [Relational Expression 2]

B _(TH)=[Tensile Strength (TS, MPa)]²*[Hole Expansion Ratio (HER,%)]^(1/2); and  [Relational Expression 3]

I _(YR)=1−[Yield Ratio (YR)].  [Relational Expression 4]

In an aspect of the present disclosure, a method for manufacturing ahigh strength steel sheet having excellent workability may comprise:providing a cold-rolled steel sheet including, by wt %, C: 0.1 to 0.25%,Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less,S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance ofFe, and unavoidable impurities; heating (primarily heating) thecold-rolled steel sheet to a temperature of 700° C. at an averageheating rate of 5° C./s or more, heating (secondarily heating) theprimarily heated steel sheet to a temperature within a range of Ac3 to920° C. at an average heating rate of 5° C./s or less, and thenmaintaining (primarily maintaining) the secondarily heated steel sheetfor 50 to 1200 seconds; cooling (primarily cooling) the primarilymaintained steel sheet to a temperature within a range of 200 to 400° C.at an average cooling rate of 1° C./s or more; heating (tertiarilyheating) the primarily cooled steel sheet to a temperature within arange of 350 to 550° C. at an average heating rate of 5° C./s or more,and then maintaining (secondarily maintaining) the tertiarily heatedsteel sheet for 50 seconds or more; and cooling (secondarily cooling)the secondarily maintained steel sheet to room temperature at an averagecooling rate of 1° C./s or more.

The steel slab may further comprise, by wt %, one or more of thefollowing (1) to (8):

-   -   (1) one or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to        0.5%;    -   (2) one or more of Cr: 0 to 3.0% and Mo: 0 to 3.0%;    -   (3) one or more of Cu: 0 to 4.0% and Ni: 0 to 4.0%    -   (4) one or more of Ca: 0 to 0.05%, REM: 0 to 0.05% excluding Y,        and Mg: 0 to 0.05%;    -   (5) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%;    -   (6) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%;    -   (7) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%; and    -   (8) Co: 0 to 1.5%.

The cold-rolled steel sheet may be provided by: heating a steel slab to1000 to 1350° C.; performing finishing hot rolling at a temperaturewithin a range of 800 to 1000° C.; coiling the hot-rolled steel sheet ata temperature within a range of 350 to 600° C.; pickling the coiledsteel sheet; and cold rolling the pickled steel sheet at a reductionratio of 30 to 90%.

Advantageous Effects

According to an aspect of the present disclosure, it is possible toprovide a steel sheet that may be used for automobile parts and thelike, and to a steel sheet having an excellent balance of tensilestrength and ductility, an excellent balance of tensile strength andhole expansion ratio, and an excellent yield ratio evaluation index, anda method for manufacturing the same.

BEST MODE

The present disclosure relates to a high strength steel sheet havingexcellent workability and a method for manufacturing the same, andexemplary embodiments of the present disclosure will hereinafter bedescribed. Exemplary embodiments of the present disclosure may bemodified into various forms, and it is not to be interpreted that thescope of the present disclosure is limited to exemplary embodimentsdescribed below. The present exemplary embodiments are provided in orderto further describe the present disclosure in detail to those skilled inthe art to which the present disclosure pertains.

The inventors of the present disclosure recognized that in a boron(B)-added transformation-induced plasticity (TRIP) steel comprisingbainite, tempered martensite, fresh martensite and retained austenite,when the fractions of the tempered martensite, the fresh martensite, andthe retained austenite are controlled to be within certain ranges, thecontents of the boron (B) contained in the tempered martensite and thefresh martensite are controlled to be within certain ranges, and a shapeand a size of the retained austenite are controlled to be within certainranges, it is possible to simultaneously secure an excellent balance oftensile strength and ductility, an excellent balance of tensile strengthand a hole expansion ratio, and an excellent yield ratio evaluationindex. Based thereon, the present inventors have conceived of thepresent disclosure by devising a method such that excellent strength,excellent yield ratio, excellent ductility, and an excellent holeexpansion ratio may be simultaneously provided.

Hereinafter, a high strength steel sheet having excellent workabilityaccording to an aspect of the present disclosure will be described inmore detail.

In an aspect of the present disclosure, a high strength steel sheethaving excellent workability may comprise: by wt %, C: 0.1 to 0.25%, Si:0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S:0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe,and unavoidable impurities; and as microstructures, bainite, temperedmartensite, fresh martensite, retained austenite and unavoidablestructures, wherein the high strength steel sheet may satisfy thefollowing relational expression 1:

0.03≤[B] _(FM) /[B] _(TM)≤0.55  [Relational Expression 1]

-   -   where [B]_(FM) is a content (wt %) of boron (B) contained in the        fresh martensite, and [B]_(TM) is a content (wt %) of boron (B)        contained in the tempered martensite.

Hereinafter, compositions of steel according to the present disclosurewill be described in more detail. Hereinafter, unless otherwiseindicated, % indicating a content of each element is based on weight.

The high strength steel sheet having excellent workability according toan aspect of the present disclosure includes: by wt %, C: 0.1 to 0.25%,Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less,S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance ofFe, and unavoidable impurities. In addition, the high strength steelsheet may further include one or more of Ti: or less (including 0%), Nb:0.5% or less (including 0%), V: 0.5% or less (including 0%), Cr: 3.0% orless (including 0%), Mo: 3.0% or less (including 0%), Cu: 4.0% or less(including 0%), Ni: 4.0% or less (including 0%), Ca: 0.05% or less(including 0%), REM: 0.05% or less (including 0%) excluding Y, Mg: 0.05%or less (including 0%), W: 0.5% or less (including 0%), Zr: 0.5% or less(including 0%), Sb: or less (including 0%), Sn: 0.5% or less (including0%), Y: 0.2% or less (including 0%), Hf: 0.2% or less (including 0%),and Co: 1.5% or less (including 0%).

Carbon (C): 0.1 to 0.25%

Carbon (C) is an unavoidable element for securing strength of a steelsheet, and is also an element for stabilizing retained austenite thatcontributes to the improvement in ductility of the steel sheet.Accordingly, in the present disclosure, 0.1% or more of carbon (C) maybe added in order to achieve such an effect. A preferable content ofcarbon (C) may be greater than 0.1%, may be 0.11% or more, and may be0.12% or more. On the other hand, when the content of carbon (C) exceedsa certain level, ductility may be lowered and weldability may bedegraded due to an excessive increase in strength. Therefore, an upperlimit of the content of carbon (C) of the present disclosure may belimited to 0.25%. The content of carbon (C) may be 0.24% or less, and amore preferable content of carbon (C) may be or less.

Silicon (Si): 0.01 to 1.5% or Less

Silicon (Si) is an element that contributes to improvement in strengthby solid solution strengthening, and is also an element improvingworkability by homogenizing a structure. In addition, silicon (Si) is anelement contributing to generation of retained austenite by suppressingprecipitation of cementite. Therefore, in the present disclosure,silicon (Si) of 0.01% or more may be added in order to achieve such aneffect. A preferable content of silicon (Si) may be 0.02% or more, and amore preferable content of silicon (Si) may be 0.04% or more. However,when the content of silicon (Si) exceeds a certain level, a problem ofplating defects, such as non-plating, may be induced during plating, andweldability of a steel sheet may be lowered, so the present disclosuremay limit an upper limit of the content of silicon (Si) to 1.5%. Apreferable upper limit of the content of silicon (Si) may be 1.48%, anda more preferable upper limit of the content of silicon (Si) may be1.46%.

Manganese (Mn): 1.0 to 4.0%

Manganese (Mn) is a useful element for increasing both strength andductility. Therefore, in the present disclosure, manganese (Mn) of 1.0%or more may added in order to achieve such an effect. A preferable lowerlimit of the content of manganese (Mn) may be 1.2%, and a morepreferable lower limit of the content of manganese (Mn) may be 1.4%. Onthe other hand, when manganese (Mn) is excessively added, a bainitetransformation time increases and concentration of carbon (C) inaustenite becomes insufficient, so there exists a problem in which adesired austenite fraction may not be secured. Therefore, in the presentdisclosure, an upper limit of the content of manganese (Mn) of thepresent disclosure may be limited to 4.0%. A preferable upper limit ofthe content of manganese (Mn) may be 3.9%.

Aluminum (Al): 0.01 to 1.5%

Aluminum (Al) is an element performing deoxidation by combining withoxygen in steel. In addition, aluminum (Al) is also an element forstabilizing retained austenite by suppressing precipitation of cementitelike silicon (Si). Therefore, in the present disclosure, aluminum (Al)of 0.01% or more may be added in order to achieve such an effect. Apreferable content of aluminum (Al) may be 0.03% or more, and a morepreferable content of aluminum (Al) may be 0.05% or more. On the otherhand, when aluminum (Al) is excessively added, inclusions in a steelsheet increase, and workability of the steel sheet may be lowered, sothe present disclosure may limit an upper limit of the content ofaluminum (Al) to 1.5%. A preferable upper limit of the content ofaluminum (Al) may be 1.48%.

Phosphorus (P): 0.15% or Less (Including 0%)

Phosphorus (P) is an element which is contained as an impurity anddeteriorates impact toughness. Therefore, it is preferable to manage thecontent of phosphorus (P) to 0.15% or less.

Sulfur (S): 0.03% or Less (Including 0%)

Sulfur (S) is an element which is contained as an impurity to form MnSin a steel sheet and deteriorate ductility. Therefore, it is preferablethat the content of sulfur (S) is 0.03% or less.

Nitrogen (N): 0.03% or Less (Including 0%)

Nitrogen (N) is an element which is contained as an impurity and formsnitride during continuous casting to cause cracks in a slab. Therefore,it is preferable that the content of nitrogen (N) is 0.03% or less.

Boron (B): 0 to 0.005%

Boron (B) is an element improving hardenability to increase strength,and is also an element suppressing nucleation of grain boundaries. Inaddition, in the present disclosure, it is intended to simultaneouslysecure an excellent balance of tensile strength and elongation, anexcellent balance of tensile strength and a hole expansion ratio, and anexcellent yield ratio evaluation index, and therefore, boron (B) is tobe necessarily added in the present disclosure. Therefore, in thepresent disclosure, 0.0005% or more of boron (B) may be added in orderto achieve such an effect. However, when the content of boron (B) isadded beyond a certain level, not only excessive characteristic effects,but also an increase in manufacturing costs is incurred, so the presentdisclosure may limit an upper limit of the content of boron (B) to0.005%.

Meanwhile, the steel sheet of the present disclosure has an alloycomposition that may be additionally included in addition to theabove-described alloy components, which will be described in detailbelow.

One or More of Titanium (Ti): 0 to 0.5%, Niobium (Nb): 0 to 0.5%, andVanadium (V): 0 to 0.5%

Titanium (Ti), niobium (Nb), and vanadium (V) are elements that makeprecipitates and refine crystal grains, and are elements that alsocontribute to the improvement in strength and impact toughness of asteel sheet, and therefore, in the present disclosure, one or more oftitanium (Ti), niobium (Nb), and vanadium (V) may be added in order toachieve such an effect. However, when each of the contents of titanium(Ti), niobium (Nb), and vanadium (V) exceeds a certain level, excessiveprecipitates are formed to lower impact toughness and increasemanufacturing costs, so the present disclosure may limit the contents oftitanium (Ti), niobium (Nb), and vanadium (V) to 0.5% or less,respectively.

One or More of Chromium (Cr): 0 to 3.0% and Molybdenum (Mo): 0 to 3.0%

Since chromium (Cr) and molybdenum (Mo) are elements that not onlysuppress austenite decomposition during alloying treatment, but alsostabilize austenite like manganese (Mn), in the present disclosure, oneor more of chromium (Cr) and molybdenum (Mo) may be added in order toachieve such an effect. However, when the contents of chromium (Cr) andmolybdenum (Mo) exceed certain levels, the bainite transformation timeincreases and the concentration of carbon (C) in austenite becomesinsufficient, so the desired retained austenite fraction may not besecured. Therefore, the present disclosure may limit the contents ofchromium (Cr) and molybdenum (Mo) to 3.0% or less, respectively.

One or More of Cu: 0 to 4.0% and Ni: 0 to 4.0%

Copper (Cu) and nickel (Ni) are elements that stabilize austenite andsuppress corrosion. In addition, copper (Cu) and nickel (Ni) are alsoelements that are concentrated on a surface of a steel sheet to preventhydrogen from intruding into the steel sheet, thereby suppressinghydrogen delayed destruction. Therefore, in the present disclosure, oneor more of copper (Cu) and nickel (Ni) may be added in order to achievesuch an effect. However, when the contents of copper (Cu) and nickel(Ni) exceed certain levels, not only excessive characteristic effects,but also an increase in manufacturing costs is incurred, so the presentdisclosure may limit the contents of copper (Cu) and nickel (Ni) to 4.0%or less, respectively.

One or More of Calcium (Ca): 0 to 0.05%, Magnesium (Mg): 0 to 0.05%, andRare Earth Element (REM) Excluding Yttrium (Y): 0 to 0.05%

Here, the rare earth element (REM) is scandium (Sc), yttrium (Y), and alanthanide element. Since calcium (Ca), magnesium (Mg), and the rareearth element (REM) excluding yttrium (Y) are elements that contributeto the improvement in ductility of a steel sheet by spheroidizingsulfides, in the present disclosure, one or more of calcium (Ca),magnesium (Mg), and the rare earth element (REM) excluding yttrium (Y)may be added in order to achieve such an effect. However, when thecontents of calcium (Ca), magnesium (Mg), and the rare earth element(REM) excluding yttrium (Y) exceed certain levels, not only excessivecharacteristic effects, but also an increase in manufacturing costs isincurred, so the present disclosure may limit the contents of calcium(Ca), magnesium (Mg), and the rare earth element (REM) excluding yttrium(Y) to 0.05% or less, respectively.

One or More of Tungsten (W): 0 to 0.5% and Zirconium (Zr): 0 to 0.5%

Since tungsten (W) and zirconium (Zr) are elements that increasestrength of a steel sheet by improving hardenability, in the presentdisclosure, one or more of tungsten (W) and zirconium (Zr) may be addedin order to achieve such an effect. However, when the contents oftungsten (W) and zirconium (Zr) exceed certain levels, not onlyexcessive characteristic effects, but also an increase in manufacturingcosts is incurred, so the present disclosure may limit the contents oftungsten (W) and zirconium (Zr) to or less, respectively.

One or More of Antimony (Sb): 0 to 0.5% and Tin (Sn): to 0.5%

Since antimony (Sb) and tin (Sn) are elements that improve platingwettability and plating adhesion of a steel sheet, in the presentdisclosure, one or more of antimony (Sb) and tin (Sn) may be added inorder to achieve such an effect. However, when the contents of antimony(Sb) and tin (Sn) exceed certain levels, brittleness of a steel sheetincreases, and thus, cracks may occur during hot working or coldworking, so the present disclosure may limit the contents of antimony(Sb) and tin (Sn) to 0.5% or less, respectively.

One or More of Yttrium (Y): 0 to 0.2% and Hafnium (Hf): 0 to 0.2%

Since yttrium (Y) and hafnium (Hf) are elements that improve corrosionresistance of a steel sheet, in the present disclosure, one or more ofthe yttrium (Y) and hafnium (Hf) may be added in order to achieve suchan effect. However, when the contents of yttrium (Y) and hafnium (Hf)exceed certain levels, ductility of the steel sheet may deteriorate, sothe present disclosure may limit the contents of yttrium (Y) and hafnium(Hf) to 0.2% or less, respectively.

Cobalt (Co): 0 to 1.5%

Since cobalt (Co) is an element that promotes a bainite transformationto increase a TRIP effect, in the present disclosure, cobalt (Co) may beadded in order to achieve such an effect. However, when the content ofcobalt (Co) exceeds a certain level, since weldability and ductility ofa steel sheet may deteriorate, the present disclosure may limit thecontent of cobalt (Co) to 1.5% or less.

The high strength steel sheet having excellent workability according toan aspect of the present disclosure may include a balance of Fe andother unavoidable impurities in addition to the components describedabove. However, in a general manufacturing process, unintendedimpurities may inevitably be mixed from raw materials or the surroundingenvironment, and thus, these impurities may not be completely excluded.Since these impurities are known to those skilled in the art, all thecontents are not specifically mentioned in the present specification. Inaddition, a further addition of effective components other than theabove-described components is not entirely excluded.

The high strength steel sheet having excellent workability according toan aspect of the present disclosure may include, as microstructures,bainite, tempered martensite, fresh martensite, retained austenite andunavoidable structures.

Both untempered martensite (fresh martensite, FM) and temperedmartensite (TM) are microstructures that improve the strength of a steelsheet. However, compared with tempered martensite, fresh martensite hasa characteristic of greatly reducing ductility and burring workabilityof a steel sheet. In addition, compared with tempered martensite, freshmartensite has a tendency of reducing a yield ratio of a steel sheet.These are because a microstructure of tempered martensite is softened bya tempering heat treatment. Therefore, in the present disclosure, it ispreferable to control fractions of tempered martensite and freshmartensite in order to secure a balance (TS²*EL^(1/2)) of tensilestrength and elongation, a balance (TS²*HER^(1/2)) of tensile strengthand a hole expansion ratio, and a yield ratio evaluation index (1-YR)targeted by the present disclosure. In order to satisfy a balance(TS²*EL^(1/2)) of tensile strength and elongation of 3.0*10⁶ or more, abalance (TS²*HER^(1/2)) of tensile strength and a hole expansion ratioof 6.0*10⁶ or more, and a yield ratio evaluation index (1-YR) of 0.42 orless, it is preferable to limit a fraction of the tempered martensite to50 vol % or more, and to limit a fraction of the fresh martensite to 10vol % or more. A more preferable fraction of tempered martensite may be52 vol % or more, or 54 vol % or more, and a more preferable fraction offresh martensite may be 12 vol % or more. On the other end, whentempered martensite or fresh martensite is excessively formed, ductilityand burring workability are lowered, so that a balance (TS²*EL^(1/2)) oftensile strength and elongation of 3.0*10⁶ or more, a balance(TS²*HER^(1/2)) of tensile strength and a hole expansion ratio of6.0*10⁶ or more, and a yield ratio evaluation index (1-YR) of 0.42 orless cannot be satisfied at the same time. Therefore, the presentdisclosure may limit a fraction of tempered martensite to 70 vol % orless, and limit a fraction of fresh martensite to 30 vol % or less. Amore preferable fraction of tempered martensite may be 68 vol % or less,or 65 vol % or less, and more preferable fraction of fresh martensitemay be 25 vol % or less.

It is necessary to optimize a fraction of bainite in order to secure abalance (TS²*EL^(1/2)) of tensile strength and elongation, a balance(TS²*HER^(1/2)) of tensile strength and a hole expansion ratio, and ayield ratio evaluation index (1-YR) at levels targeted by the presentdisclosure. In order to secure a balance (TS²*EL^(1/2)) of tensilestrength and elongation of 3.0*10⁶ or more, a balance (TS²*HER^(1/2)) oftensile strength and a hole expansion ratio of 6.0*10⁶ or more, and ayield ratio evaluation index (1-YR) of 0.42 or less, it is preferable tocontrol a fraction of bainite to 10 vol % or more. A more preferablefraction of bainite may be 12 vol % or more, or 14 vol % or more. On theother end, when bainite is excessively formed, it causes a fractionreduction of tempered martensite, so that a fraction of bainite may belimited to 30 vol % or less, in order to secure a balance (TS²*EL^(1/2))of tensile strength and elongation, a balance (TS²*HER^(1/2)) of tensilestrength and a hole expansion ratio, and a yield ratio evaluation index(1-YR) targeted by the present disclosure. A preferable fraction of thebainite may be 12 vol % or more, 14 vol % or more, 28 vol % or less, or26 vol % or less.

A steel sheet including retained austenite has excellent ductility andbending workability due to transformation-induced plasticity occurringduring transformation from austenite to martensite during processing.When a fraction of the retained austenite is lower than a certain level,a balance (TS²*EL^(1/2)) of tensile strength and elongation may be lessthan 3.0*10⁶ (Mpa²%^(1/2)), and is not preferably. On the other hand,when a fraction of retained austenite exceeds a certain level, localelongation may be lowered, or point weldability may be lowered.Therefore, in the present disclosure, a fraction of retained austenitemay be limited to be in a range of 2 to 10 vol % in order to obtain asteel sheet having an excellent balance (TS²*EL^(1/2)) of tensilestrength and elongation. A preferable fraction of retained austenite is3 vol % or more, or 8 vol % or less.

As the unavoidable structure, the steel sheet of the present disclosuremay include ferrite, pearlite, martensite austenite constituent (M-A),and the like. When ferrite is excessively formed, strength of the steelsheet may be lowered, so the present disclosure may limit a fraction offerrite to 5 vol % (including 0%). Moreover, when pearlite isexcessively formed, workability of the steel sheet may be lowered or afraction of retained austenite may be lowered, so the present disclosureintends to limit the formation of pearlite as much as possible.

The high strength steel sheet having excellent workability according toan aspect of the present disclosure may satisfy the following relationalexpression 1:

0.03≤[B] _(FM) /[B] _(TM)≤0.55  [Relational Expression 1]

-   -   where [B]_(FM) is a content (wt %) of boron (B) contained in the        fresh martensite, and [B]_(TM) is a content (wt %) of boron (B)        contained in the tempered martensite.

The present disclosure not only controls the fractions of the temperedmartensite, the fresh martensite, and the retained austenite to be incertain ranges, but also controls the content ratios of the boron (B)contained in the tempered martensite, and the fresh martensite to be incertain ranges, while controlling a ratio of the retained austenite ofspecific size, shape and type with respect to the entire retainedaustenite to be in a certain range, in order to secure a balance(TS²*EL^(1/2)) of tensile strength and elongation, a balance(TS²*HER^(1/2)) of tensile strength and a hole expansion ratio, and ayield ratio evaluation index (1-YR) targeted thereby.

The present disclosure controls a ratio of the content ([B]_(TM), wt %)of boron (B) contained in the fresh martensite to the content ([B]_(FM),wt %) of boron (B) contained in the tempered martensite to be in a rangeof 0.03 to 0.55 as shown in relational expression 1, so as to secure abalance (B_(TE)) of tensile strength and elongation of 3.0*10⁶ to6.2*10⁶ (mpa²%^(1/2)), a balance (B_(TH)) of tensile strength and a holeexpansion ratio of 6.0*10⁶ to 11.5*10⁶ (Mpa²%^(1/2)), and a yield ratioevaluation index (I_(YR)) of 0.15 to 0.42 at the same time.

The inventors of the present disclosure conducted in-depth research on amethod for securing physical properties of a boron (B)-added TRIP steal,and as a result, noted that the physical properties targeted by thepresent disclosure may be secured only when a ratio of a content ofboron (B) contained in fresh martensite to a content of boron (B)contained in tempered martensite satisfies a certain range, even thoughthe theoretical basis thereof is not clearly identified. In particular,it was able to identify that a yield ratio of a steel sheet has aconstant tendency according to a content ratio of boron (B) contained intempered martensite and fresh martensite. Therefore, the presentdisclosure limits the ratio of the content of boron (B) contained in thefresh martensite to the content of boron (B) contained in the temperedmartensite to be in a range of 0.03 to 0.55 as shown in relationalexpression 1, thereby securing a balance (TS²*EL^(1/2)) of tensilestrength and elongation, a balance (TS²*HER^(1/2)) of tensile strengthand a hole expansion ratio, and a yield ratio evaluation index (1-YR)targeted thereby.

In the high strength steel sheet having excellent workability accordingto an aspect of the present disclosure, a balance (B_(TE)) of tensilestrength and elongation expressed by the following relational expression2 may satisfy 3.0*10⁶ to 6.2*10⁶ (Mpa²%^(1/2)), a balance (B_(TH)) oftensile strength and a hole expansion ratio expressed by the followingrelational expression 3 may satisfy 6.0*10⁶ to 11.5*10⁶ (Mpa²%^(1/2)),and a yield ratio evaluation index (I_(YR)) expressed by the followingrelational expression 4 may satisfy 0.15 to 0.42:

B _(TE)=[Tensile Strength (TS, MPa)]²*[Elongation (El,%)]^(1/2h);  [Relational Expression 2]

B _(TH)=[Tensile Strength (TS, MPa)]²*[Hole Expansion Ratio (HER,%)]^(1/2); and  [Relational Expression 3]

I _(YR)=1−[Yield Ratio (YR)].  [Relational Expression 4]

Hereinafter, an example of a method for manufacturing a steel sheet ofthe present disclosure will be described in detail.

A method for manufacturing a high strength steel sheet having excellentworkability according to an aspect of the present disclosure maycomprises: heating (primarily heating) a cold-rolled steel sheet havinga predetermined alloy composition to a temperature of 700° C. at anaverage heating rate of 5° C./s or more, heating (secondarily heating)the primarily heated steel sheet to a temperature within a range of Ac3to 920° C. at an average heating rate of 5° C./s or less, and thenmaintaining (primarily maintaining) the secondarily heated steel sheetfor 50 to 1200 seconds; cooling (primarily cooling) the primarilymaintained steel sheet to a temperature within a range of 200 to 400° C.at an average cooling rate of 1° C./s or more; heating (tertiarilyheating) the primarily cooled steel sheet to a temperature within arange of 350 to 550° C. at an average heating rate of 5° C./s or more,and then maintaining (secondarily maintaining) the tertiarily heatedsteel sheet for 50 seconds or more; and cooling (secondarily cooling)the secondarily maintained steel sheet to room temperature at an averagecooling rate of 1° C./s or more.

The cold-rolled steel sheet may be provided by: heating steel slabhaving a predetermined alloy composition to 1000 to 1350° C.; performingfinishing hot rolling at a temperature within a range of 800 to 1000°C.; coiling the hot-rolled steel sheet at a temperature within a rangeof 350 to 600° C.; pickling the coiled steel sheet; and cold rolling thepickled steel sheet at a reduction ratio of 30 to 90%.

Preparation and Heating of Steel Slab

A steel slab having a predetermined alloy composition is prepared. Sincethe steel slab according to the present disclosure includes an alloycomposition corresponding to an alloy composition of the steel sheetdescribed above, the description of the alloy compositions of the slabis replaced by the description of the alloy composition of the steelsheet described above.

The prepared steel slab may be heated to a temperature within a certainrange, and the heating temperature of the steel slab at this time may bein the range of 1000 to 1350° C. When the heating temperature of thesteel slab is less than 1000° C., the steel slab may be hot rolled at atemperature within a range below a desired finish hot rollingtemperature range, and when the heating temperature of the steel slabexceeds 1350° C., the temperature reaches a melting point of steel, andthus, the steel slab may be melted.

Hot Rolling and Coiling

The heated steel slab may be hot rolled, and thus, provided as ahot-rolled steel sheet. During the hot rolling, the finish hot rollingtemperature is preferably in the range of 800 to 1000° C. When thefinish hot rolling temperature is lower than 800° C., an excessiverolling load may be a problem, and when the finish hot rollingtemperature exceeds 1000° C., grains of the hot-rolled steel sheet arecoarsely formed, which may cause a deterioration in physical propertiesof the final steel sheet.

After the hot rolling has been completed, the hot-rolled steel sheet maybe cooled at an average cooling rate of 10° C./s or more, and may becoiled at a temperature within a range of 350 to 650° C. When thecoiling temperature is lower than 350° C., coiling is not easy, and whenthe coiling temperature exceeds 650° C., surface scale may be formedinto the inside of the hot-rolled steel sheet, which may make picklingdifficult.

Pickling and Cold Rolling

After uncoiling the coiled hot-rolled coil, in order to remove the scalegenerated on the surface of the steel sheet, the pickling may beperformed, and the cold rolling may be performed. Although theconditions of the pickling and the cold rolling are not particularlylimited in the present disclosure, the cold rolling is preferablyperformed at a cumulative reduction ratio of 30 to 90%. When thecumulative reduction ratio of the cold rolling exceeds 90%, it may bedifficult to perform the cold rolling in a short time due to the highstrength of the steel sheet.

The cold-rolled steel sheet may be manufactured as a non-platedcold-rolled steel sheet through the annealing heat treatment process, ormay be manufactured as a plated steel sheet through a plating process toimpart corrosion resistance. As the plating, plating methods such ashot-dip galvanizing, electro-galvanizing, and hot-dip aluminum platingmay be applied, and the method and the type are not particularlylimited.

Annealing Heat Treatment

In the present disclosure, in order to simultaneously secure thestrength and workability of the steel sheet, the annealing heattreatment process is performed.

The cold-rolled steel sheet is heated (primarily heated) to atemperature of 700° C. at an average heating rate of 5° C./s or more, isheated (secondarily heated) to a temperature within a range of Ac3 to920° C. at an average heating rate of 5° C./s or less, and then ismaintained (primarily maintained) for 50 to 1200 seconds.

When the average heating rate to a temperature of 700° C. of the primaryheating is less than 5° C./s, lump austenite is formed from ferrite andcementite generated during heating, and as a result, fine temperedmartensite and retained austenite cannot be formed as a final structure.Therefore, a targeted balance (TS²*EL^(1/2)) of tensile strength andelongation, and a targeted balance (TS²*HER^(1/2)) of tensile strengthand a hole expansion ratio cannot be implemented. In addition, when thesecondary heating rate up to the primary maintaining temperature exceeds5° C./s, transformation from cementite generated during heating toaustenite is accelerated, so that a large amount of lump austenite isformed, the final structure is coarsened, and boron (B) may not besufficiently concentrated into tempered martensite. As a result,[B]_(FM)/[B]_(TM) exceeds 0.55, and targeted levels of a balance(TS²*EL^(1/2)) of tensile strength and elongation and a balance(TS²*HER^(1/2)) of tensile strength and a hole expansion ratio, and ayield ratio evaluation index (I_(YR)) cannot be implemented.

When the primary maintaining temperature is lower than Ac3 (two-phaseregion), 5 vol % or more of ferrite is formed, and therefore, a balance(TS²*EL^(1/2)) of tensile strength and elongation and a balance(TS²*HER^(1/2)) of tensile strength and a hole expansion ratio may belowered. In addition, when the primary maintaining time is less than 50seconds, the structure may not be sufficiently homogenized and thephysical properties of the steel sheet may be lowered. Upper limits ofthe primary maintaining temperature and the primary maintaining time arenot particularly limited, but it is preferable that the primarymaintaining temperature is limited to 920° C. or less, and the primarymaintaining time is limited to 1200 seconds or less, in order to preventtoughness reduction due to coarsened grains.

After the primary maintaining, the primarily maintained steel sheet maybe cooled (primarily cooled) to a primary cooling stop temperature in arange of 200 to 400° C. at an average cooling rate of 1° C./s or more.When the average cooling rate of the primary cooling is less than 1°C./s, a fraction of retained austenite becomes insufficient due to aslow cooling, and therefore, a balance (TS²*EL^(1/2)) of tensilestrength and elongation of the steel sheet may be lowered. An upperlimit of the average cooling rate of the primary cooling does not needto be particularly specified, but is preferably set to 100° C./s orless. When the primary cooling stop temperature is lower than 200° C.,tempered martensite is excessively formed, and retained austenitebecomes insufficient, whereby a balance (TS²*EL^(1/2)) of tensilestrength and elongation and a balance (TS²*HER^(1/2)) of tensilestrength and a hole expansion ratio of the steel sheet may be lowered.On the other hand, when the primary cooling stop temperature exceeds400° C., bainite is excessively formed, and tempered martensite becomesinsufficient, whereby a balance (TS²*EL^(1/2)) of tensile strength andelongation and a balance (TS²*HER^(1/2)) of tensile strength and a holeexpansion ratio of the steel sheet may be lowered.

After the secondary cooling, the primarily cooled steel sheet may beheated (tertiarily heated) to a temperature within a range of 350 to550° C. at an average heating rate of 5° C./s or more, and then may bemaintained (secondarily maintaining) for 50 seconds or more. An upperlimit of the average heating rate of the tertiary heating does not needto be particularly specified, but is preferably set to 100° C./s orless. When the secondary maintaining temperature is lower than 350° C.or the secondary maintaining time is less than 50 seconds, tamperedmartensite is excessively formed, and therefore, it is difficult tosecure a fraction of retained austenite. As a result, a balance(TS²*EL^(1/2)) of tensile strength and elongation and a balance(TS²*HER^(1/2)) of tensile strength and a hole expansion ratio may belowered. When the secondary maintaining temperature exceeds 550° C. orthe secondary maintaining time exceeds 155,000 seconds, a fraction ofretained austenite becomes insufficient, and therefore, a balance(TS²*EL^(1/2)) of tensile strength and elongation of the steel sheet maybe lowered.

After the secondary maintaining, the secondarily maintained steel sheetmay be cooled (secondarily cooling) to room temperature at an averagecooling rate of 1° C./s or more.

The high strength steel sheet having excellent workability manufacturedby the aforementioned manufacturing method may comprise, asmicrostructures, bainite, tempered martensite, fresh martensite,retained austenite and unavoidable structures, and as a preferableexample, may comprise, by volume fraction, 10 to 30% of bainite, 50 to70% of tempered martensite, 10 to 30% of fresh martensite, 2 to 10% ofretained austenite, and 5% or less (including 0%) of ferrite.

In the high strength steel sheet having excellent workabilitymanufactured by the aforementioned manufacturing method, a balance(B_(TE)) of tensile strength and elongation expressed by the followingrelational expression 2 may satisfy 3.0*10⁶ to 6.2*10⁶ (MPa²%^(1/2)), abalance (B_(TH)) of tensile strength and a hole expansion ratioexpressed by the following relational expression 3 may satisfy 6.0*10⁶to 11.5*10⁶ (MPa²%^(1/2)), and a yield ratio evaluation index (I_(YR))expressed by the following relational expression 4 may satisfy 0.15 to0.42:

B _(TE)=[Tensile Strength (TS, MPa)]²*[Elongation (El,%)]^(1/2);  [Relational Expression 2]

B _(TH)=[Tensile Strength (TS, MPa)]²*[Hole Expansion Ratio (HER,%)]^(1/2); and  [Relational Expression 3]

I _(YR)=1−[Yield Ratio (YR)].  [Relational Expression 4]

Mode for Invention

Hereinafter, a high strength steel sheet having excellent workabilityand a method for manufacturing same according to an aspect of thepresent disclosure will be described in more detail. It should be notedthat the following examples are only for the understanding of thepresent disclosure, and are not intended to specify the scope of thepresent disclosure. The scope of the present disclosure is determined bymatters described in claims and matters reasonably inferred therefrom.

Inventive Examples

A steel slab having a thickness of 100 mm having alloy compositions (abalance of Fe and unavoidable impurities) shown in Table 1 below wasprepared, heated at 1200° C., and then subjected to finish hot rollingat 900° C. Thereafter, the steel slab was cooled at an average coolingrate of 30° C./s, and coiled at a coiling temperature of Tables 2 and 3to manufacture a hot-rolled steel sheet having a thickness of 3 mm.Thereafter, after removing a surface scale by pickling, cold rolling wasperformed to a thickness of 1.5 mm.

Thereafter, the heat treatment was performed under the annealing heattreatment conditions shown in Tables 2 to 5 to manufacture the steelsheet. In Tables 2 and 3, the single-phase region means a temperaturerange of Ac3 to 920° C., and the two-phase region means a temperaturerange below Ac3° C.

The microstructure of the thus prepared steel sheet was observed, andthe results were shown in Tables 6 and 7. Among the microstructures,ferrite (F), bainite (B), tempered martensite (TM), fresh martensite(FM) and pearlite (P) were observed through SEM after nital-etching apolished specimen cross section. After nital-etching, a structure havingno concave-convex portions on a surface of a specimen was classified asferrite, and a structure having a lamella structure of cementite andferrite is classified as pearlite. Since both of bainite (B) andtempered martensite (TM) were observed in a form of lath and block, andwere difficult to distinguish each other, the fractions of bainite andtempered martensite were calculated using an expansion curve afterevaluating dilatation. That is, a value obtained by subtracting thefraction of tempered martensite calculated using the expansion curvefrom the fraction of bainite and tempered martensite measured by the SEMobservation was determined as the fraction of bainite. Meanwhile, sincefresh martensite (FM) and retained austenite (retained γ) are alsodifficult to distinguish each other, a value obtained by subtracting thefraction of retained austenite calculated by an X-ray diffraction methodfrom the fraction of martensite and retained austenite observed by theSEM was determined as the fraction of the fresh martensite.

Meanwhile, [B]_(FM)/[B]_(TM), a balance (TS²*EL^(1/2)) of tensilestrength and elongation, a balance (TS²*HER^(1/2)) of tensile strengthand a hole expansion ratio, and a yield ratio evaluation index (I_(YR))of the steel sheet were measured and evaluated, and the results thereofwere shown in Tables 8 and 9.

The concentrations of boron (B) in fresh martensite and temperedmartensite measured using an electron probe microanalyser (EPMA) weredetermined as a content ([B]_(FM)) of boron (B) contained in freshmartensite, and a content ([B]_(TM)) of boron (B) contained in temperedmartensite.

Tensile strength (TS) and elongation (El) were evaluated through atensile test, and the tensile strength (TS) and the elongation (El) weremeasured by evaluating the specimens collected in accordance with JISNo. 5 standard based on a 90° direction with respect to a rollingdirection of a rolled sheet. The hole expansion ratio (HER) wasevaluated through a hole expansion test, and was calculated by thefollowing relational expression 5, after forming a punching hole (dieinner diameter of 10.3 mm, clearance of 12.5%) of 10 mmψ, inserting aconical punch having an apex angle of 60° into a punching hole in adirection in which a burr of the punching hole faces outward, and thencompressing and expanding a peripheral portion of the punching hole at amoving speed of 20 mm/min:

Hole Expansion Ratio (HER, %)={(D−D ₀)/D ₀}×100  [Relational Expression5]

In the above relational expression 5, D is a hole diameter (mm) whencracks penetrate through the steel sheet along the thickness direction,and D₀ is the initial hole diameter (mm).

TABLE 1 Steel Chemical Components (wt %) Type C Si Mn P S Al N Cr Mo BOthers A 0.14 0.57 2.63 0.011 0.0008 0.33 0.0032 0.0026 B 0.15 0.41 2.150.012 0.0010 0.55 0.0028 0.28 0.24 0.0023 C 0.13 0.62 2.02 0.009 0.00110.47 0.0030 0.48 0.0021 D 0.22 0.75 1.22 0.011 0.0011 0.87 0.0026 0.850.0025 E 0.17 1.42 2.08 0.010 0.0009 0.13 0.0033 0.0047 F 0.14 0.15 1.880.009 0.0010 1.44 0.0029 0.0044 G 0.12 0.35 2.76 0.008 0.0013 0.750.0031 0.0036 Ti: 0.04 H 0.20 0.29 2.57 0.011 0.0011 0.42 0.0026 0.0023Nb: 0.05 I 0.11 0.35 2.35 0.010 0.0010 0.35 0.0029 0.0020 V: 0.04 J 0.140.38 1.84 0.009 0.0012 0.56 0.0028 0.0014 Ni: 0.32 K 0.15 0.53 2.320.012 0.0009 0.63 0.0026 0.0016 Cu: 0.39 L 0.12 0.71 2.57 0.010 0.00070.69 0.0032 0.0008 M 0.21 0.42 3.85 0.009 0.0008 0.57 0.0029 0.0007 Ca:0.002 N 0.16 0.94 2.55 0.011 0.0010 0.52 0.0035 0.0034 REM: 0.001 O 0.130.62 2.79 0.009 0.0012 0.52 0.0030 0.0036 Mg: 0.002 P 0.12 0.57 2.300.010 0.0009 0.55 0.0028 0.0031 W: 0.12 Q 0.18 0.62 2.16 0.011 0.00130.46 0.0034 0.0030 Zr: 0.13 R 0.15 0.05 2.73 0.010 0.0007 1.47 0.00320.0026 Sb: 0.02 S 0.23 1.45 2.22 0.009 0.0012 0.04 0.0031 0.0027 Sn:0.03 T 0.13 0.97 2.51 0.009 0.0009 0.47 0.0028 0.0025 Y: 0.01 U 0.170.72 2.75 0.010 0.0010 0.43 0.0032 0.0022 Hf: 0.02 V 0.14 0.66 2.550.009 0.0013 0.49 0.0029 0.0024 Co: 0.32 XA 0.08 0.48 2.23 0.008 0.00110.45 0.0033 0.0021 XB 0.27 0.52 2.07 0.009 0.0009 0.37 0.0029 0.0032 XC0.14 0.001 2.06 0.011 0.0007 0.001 0.0027 0.0035 XD 0.13 1.55 2.25 0.0100.0008 0.53 0.0035 0.0023 XE 0.17 0.65 2.28 0.012 0.0010 1.54 0.00340.0027 XF 0.15 0.59 0.83 0.009 0.0012 0.42 0.0033 0.0026 XG 0.19 0.634.21 0.008 0.0009 0.54 0.002 0.0028 XH 0.15 0.54 2.24 0.010 0.0007 0.460.0030 3.28 0.0021 XI 0.13 0.47 2.18 0.009 0.0009 0.48 0.0028 3.300.0024 XJ 0.16 0.50 2.43 0.011 0.0008 0.52 0.0025 0.0003 XK 0.14 0.532.38 0.012 0.0011 0.45 0.0027 0.0053

TABLE 2 Coiling Pri- Pri- Temp. Second- mary mary of Hot- PrimaryPrimary ary Main- Main- rolled Average Heating Average tain- tain-Speci- Steel Heating Stop Heating ing ing men Steel Sheet Rate Temp.Rate Temp. Time No. Type (° C.) (° C/s) (° C.) (° C/s) Region (s) 1 A550 15 700 0.5 Single- 180 phase Region 2 A 550 1 700 0.5 Single- 180phase Region 3 A 500 15 700 10 Single- 180 phase Region 4 A 500 15 7000.5 Two- 180 phase Region 5 A 550 15 700 0.5 Single- 180 phase Region 6A 550 15 700 0.5 Single- 180 phase Region 7 A 500 15 700 0.5 Single- 180phase Region 8 A 500 15 700 0.5 Single- 180 phase Region 9 A 550 15 7000.5 Single- 180 phase Region 10 A 550 15 700 0.5 Single- 180 phaseRegion 11 A 550 15 700 0.5 Single- 180 phase Region 12 B 500 15 700 0.5Single- 180 phase Region 13 C 500 15 700 0.5 Single- 180 phase Region 14D 500 15 700 0.5 Single- 180 phase Region 15 E 550 15 700 0.5 Single-180 phase Region 16 F 500 15 700 0.5 Single- 180 phase Region 17 G 40015 700 0.5 Single- 180 phase Region 18 H 600 15 700 0.5 Single- 180phase Region 19 I 500 15 700 0.5 Single- 180 phase Region 20 J 550 15700 0.5 Single- phase 180 Region 21 K 500 15 700 0.5 Single- phase 180Region

TABLE 3 Coiling Pri- Pri- Temp. Second- mary mary of Hot- PrimaryPrimary ary Main- Main- rolled Average Heating Average tain- tain-Speci- Steel Heating Stop Heating ing ing men Steel Sheet Rate Temp.Rate Temp. Time No. Type (° C.) (° C./s) (° C.) (° C./s) Region (s) 22 L550 15 700 0.5 Single- 180 phase Region 23 M 500 15 700 0.5 Single- 180phase Region 24 N 550 15 700 0.5 Single- 180 phase Region 25 O 500 15700 0.5 Single- 180 phase Region 26 P 550 15 700 0.5 Single- 180 phaseRegion 27 Q 550 15 700 0.5 Single- 180 phase Region 28 R 500 15 700 0.5Single- 180 phase Region 29 S 450 15 700 0.5 Single- 180 phase Region 30T 600 15 700 0.5 Single- 180 phase Region 31 U 550 15 700 0.5 Single-180 phase Region 32 V 500 15 700 0.5 Single- 180 phase Region 33 XA 55015 700 0.5 Single- 180 phase Region 34 XB 500 15 700 0.5 Single- 180phase Region 35 XC 500 15 700 0.5 Single- 180 phase Region 36 XD 500 15700 0.5 Single- 180 phase Region 37 XE 550 15 700 0.5 Single- 180 phaseRegion 38 XF 500 15 700 0.5 Single- 180 phase Region 39 XG 500 15 7000.5 Single- 180 phase Region 40 XH 550 15 700 0.5 Single- 180 phaseRegion 41 XI 550 15 700 0.5 Single- 180 phase Region 42 XJ 500 15 7000.5 Single- 180 phase Region 43 XK 500 15 700 0.5 Single- 180 phaseRegion

TABLE 4 Sec- Sec- ond- ond- Sec- ary ary ond- Primary Primary TertiaryMain- Main- ary Average Cooling Average tain- tain- Average Speci-Cooing Stop Heating ing ing Cooing men Steel Rate Temp. Rate Temp. TimeRate No. Type (° C./s) (° C.) (° C./s) (° C.) (s) (° C./s) 1 A 20 300 15400 400 10 2 A 20 300 15 400 400 10 3 A 20 350 15 450 400 10 4 A 20 30015 400 400 10 5 A 0.5 350 15 450 400 10 6 A 20 170 15 450 400 10 7 A 20430 15 400 400 10 8 A 20 300 15 320 400 10 9 A 20 350 15 580 400 10 10 A20 300 15 400 30 10 11 A 20 300 15 400 160,000 10 12 B 20 300 15 400 40010 13 C 20 350 15 500 400 10 14 D 20 350 15 450 400 10 15 E 20 250 15400 400 10 16 F 20 300 15 500 400 10 17 G 20 230 15 400 400 10 18 H 20370 15 450 400 10 19 I 20 300 15 400 400 10 20 J 20 300 15 450 400 10 21K 20 350 15 400 400 10

TABLE 5 Sec- Sec- Pri- ond- ond- Pri- mary ary ary Sec- mary Cool-Tertiary Main- Main- ondary Average ing Average tain- tain- AverageSpeci- Cooing Stop Heating ing ing Cooing men Steel Rate Temp. RateTemp. Time Rate No. Type (° C./s) (° C.) (° C./s) (° C.) (s) (° C./s) 22L 20 300 15 450 400 10 23 M 20 350 15 400 400 10 24 N 20 300 15 400 40010 25 C 20 230 15 500 400 10 26 P 20 370 15 400 400 10 27 Q 20 350 15450 400 10 28 R 20 300 15 450 400 10 29 S 20 250 15 400 400 10 30 T 20300 15 450 400 10 31 U 20 300 15 400 400 10 32 V 20 350 15 400 400 10 33XA 20 350 15 400 400 10 34 XB 20 300 15 500 400 10 35 XC 20 300 15 450400 10 36 XD 20 350 15 500 400 10 37 XE 20 300 15 450 400 10 38 XF 20300 15 400 400 10 39 XG 20 350 15 400 400 10 40 XH 20 350 15 450 400 1041 XI 20 300 15 400 400 10 42 XJ 20 350 15 400 400 10 43 XK 20 350 15400 400 10

TABLE 6 Speci- F B TM FM Y P men Steel (vol (vol (vol (vol (vol (vol No.Type %) %) %) %) %) %) 1 A 0 18 59 17 6 0 2 A 0 26 45 28 1 0 3 A 0 22 5319 4 2 4 A 17 13 52 13 5 0 5 A 0 24 57 18 1 0 6 A 0 14 73 12 1 0 7 A 035 43 16 6 0 8 A 0 14 72 13 1 0 9 A 0 21 64 15 0 0 10 A 0 15 71 13 1 011 A 0 19 62 18 1 0 12 B 0 18 58 17 7 0 13 C 0 16 60 19 5 0 14 D 0 24 5416 6 0 15 E 0 20 57 17 6 0 16 F 0 23 54 15 8 0 17 G 0 19 64 14 3 0 18 H0 15 58 18 9 0 19 I 0 17 61 17 5 0 20 J 0 19 59 15 7 0 21 K 0 18 60 16 60

TABLE 7 Speci- F B TM FM Y P men Steel (vol (vol (vol (vol (vol (vol No.Type %) %) %) %) %) %) 22 L 0 18 60 16 6 0 23 M 0 19 55 19 7 0 24 N 0 2257 15 6 0 25 O 0 23 55 17 5 0 26 P 0 24 52 15 9 0 27 Q 0 20 58 14 8 0 28R 0 16 68 13 3 0 29 S 0 18 53 25 4 0 30 T 0 23 54 16 7 0 31 U 0 20 57 158 0 32 V 0 19 58 17 6 0 33 XA 0 20 58 18 4 0 34 XB 0 15 38 34 13 0 35 XC0 18 62 19 1 0 36 XD 0 13 51 32 4 0 37 XE 0 12 52 31 5 0 38 XF 0 16 5915 1 9 39 XG 0 11 53 32 4 0 40 XH 0 13 51 33 3 0 41 XI 0 12 52 31 5 0 42XJ 0 17 62 16 5 0 43 XK 0 21 61 14 4 0

TABLE 8 Steel [B]_(FM)/ B_(TE) B_(TH) No. Type [B]_(TM) (10⁶MPa²%^(1/2)) (10⁶ MPa²%^(1/2)) 1-YR 1 A 0.34 4.6 8.3 0.28 2 A 0.17 2.44.5 0.32 3 A 0.59 1.7 5.2 0.44 4 A 0.26 2.5 4.8 0.30 5 A 0.22 2.1 7.60.33 6 A 0.31 2.5 5.3 0.29 7 A 0.34 2.2 5.0 0.25 8 A 0.23 1.6 4.4 0.28 9A 0.25 2.4 8.1 0.32 10 A 0.28 1.9 4.7 0.26 11 A 0.26 2.0 8.5 0.34 12 B0.27 3.3 9.5 0.25 13 C 0.46 6.0 10.6 0.17 14 D 0.05 4.9 10.3 0.40 15 E0.53 4.2 6.2 0.21 16 F 0.37 5.3 11.2 0.26 17 G 0.30 5.5 8.8 0.37 18 H0.09 5.5 7.5 0.35 19 I 0.49 4.8 7.1 0.20 20 J 0.28 3.4 8.9 0.24 21 K0.35 5.9 9.6 0.29

TABLE 9 Steel [B]_(FM)/ B_(TE) B_(TH) No. Type [B]_(TM) (10⁶MPa²%^(1/2)) (10⁶ MPa²%^(1/2)) 1-YR 22 L 0.35 4.1 9.4 0.22 23 M 0.30 3.38.8 0.26 24 N 0.27 4.8 7.5 0.25 25 O 0.06 5.3 8.1 0.19 26 P 0.38 5.9 8.60.30 27 Q 0.46 4.0 7.7 0.25 28 R 0.51 4.2 6.4 0.39 29 S 0.45 3.7 7.90.33 30 T 0.47 4.3 8.2 0.24 31 U 0.33 5.5 11.2 0.25 32 V 0.26 5.8 8.60.23 33 XA 0.23 2.2 4.7 0.31 34 XB 0.19 2.5 5.3 0.26 35 XC 0.12 2.8 7.60.29 36 XD 0.24 1.3 5.2 0.36 37 XE 0.20 1.8 5.4 0.31 38 XF 0.17 2.2 8.50.26 39 XG 0.31 1.8 5.4 0.31 40 XH 0.26 1.4 4.6 0.29 41 XI 0.23 2.5 5.80.33 42 XJ 0.57 3.1 6.5 0.45 43 XK 0.01 3.4 6.2 0.12

As shown in Tables 1 to 9 above, it could be seen that in the case ofthe specimens complying with the conditions presented in the presentdisclosure, relational expression 1 is satisfied, a balance (B_(TE)) oftensile strength and elongation satisfies 3.0*10⁶ to 6.2*10⁶(Mpa²%^(1/2)), a balance (B_(TH)) of tensile strength and a holeexpansion ratio satisfies 6.0*10⁶ to 11.5*10⁶ (Mpa²%^(1/2)), and a yieldratio evaluation index (I_(YR)) satisfies 0.15 to 0.42.

In specimen 2, the primary average heating rate was less than 5° C./s,so that tempered martensite and retained austenite were insufficient. Asa results, a balance (B_(TE)) of tensile strength and elongation wasless than 3.0*10⁶, and a balance (B_(TH)) of tensile strength and a holeexpansion ratio was less than 6.0*10⁶, in specimen 2.

In specimen 3, the secondary average heating rate was more than 5° C./s,so that lump austenite was formed, and boron (B) was not concentratedinto tempered martensite. As a result, [B]_(FM)/[B]_(TM) was more than0.55, a yield ratio evaluation index (I_(YR)) was more than 0.42, abalance (B_(TE)) of tensile strength and elongation was less than3.0*10⁶, and a balance (B_(TH)) of tensile strength and a hole expansionratio was less than 6.0*10⁶, in specimen 3.

In specimen 4, the primary maintaining temperature was in a two-phaseregion less than Ac3, so that a fraction of ferrite was excessive. As aresult, a balance (B_(TE)) of tensile strength and elongation was lessthan 3.0*10⁶, and a balance (B_(TH)) of tensile strength and a holeexpansion ratio was less than 6.0*10⁶, in specimen 4.

In specimen 5, the primary average cooling rate was less than 1° C./s,so that a fraction of retained austenite was insufficient. As a result,a balance (B_(TE)) of tensile strength and elongation was less than3.0*10⁶, in specimen 5.

In specimen 6, the primary cooling stop temperature was less than 200°C., so that a fraction of tempered martensite was excessive, and afraction of retained austenite was insufficient. As a result, a balance(B_(TE)) of tensile strength and elongation was less than 3.0*10⁶, and abalance (B_(TH)) of tensile strength and a hole expansion ratio was lessthan 6.0*10⁶, in specimen 6.

In specimen 7, the primary cooling stop temperature was more than 400°C., so that a fraction of bainite was excessive, and a fraction oftempered martensite was insufficient. As a result, a balance (B_(TE)) oftensile strength and elongation was less than 3.0*10⁶, and a balance(B_(TH)) of tensile strength and a hole expansion ratio was less than6.0*10⁶, in specimen 7.

In specimen 8, the secondary maintaining temperature was less than 350°C., so that a fraction of tempered martensite was excessive, and afraction of retained austenite was insufficient. As a result, a balance(B_(TE)) of tensile strength and elongation was less than 3.0*10⁶, and abalance (B_(TH)) of tensile strength and a hole expansion ratio was lessthan 6.0*10⁶, in specimen 8.

In specimen 9, the secondary maintaining temperature was more than 550°C., so that a fraction of retained austenite was insufficient. As aresult, a balance (B_(TE)) of tensile strength and elongation was lessthan 3.0*10⁶, in specimen 9.

In specimen 10, the secondary maintaining time was less than 50 seconds,so that a fraction of tempered martensite was excessive, and a fractionof retained austenite was insufficient. As a result, a balance (B_(TE))of tensile strength and elongation was less than 3.0*10⁶, and a balance(B_(TH)) of tensile strength and a hole expansion ratio was less than6.0*10⁶, in specimen 10.

In specimen 11, the secondary maintaining time was more than 155,000seconds, so that a fraction of retained austenite was insufficient. As aresult, a balance (B_(TE)) of tensile strength and elongation was lessthan 3.0*10⁶, in specimen 11.

In specimen 33, a content of carbon (C) was low, so that a balance(B_(TE)) of tensile strength and elongation was less than 3.0*10⁶, and abalance (B_(TH)) of tensile strength and a hole expansion ratio was lessthan 6.0*10⁶.

In specimen 34, a content of carbon (C) was high, so that a fraction oftempered martensite was insufficient, a fraction of fresh martensite wasexcessive, and a fraction of retained austenite was excessive. As aresult, a balance (B_(TE)) of tensile strength and elongation was lessthan 3.0*10⁶, and a balance (B_(TH)) of tensile strength and a holeexpansion ratio was less than 6.0*10⁶, in specimen 34.

In specimen 35, a content of silicon (Si) was low, so that a fraction ofretained austenite was insufficient. As a result, a balance (B_(TE)) oftensile strength and elongation was less than 3.0*10⁶, in specimen 35.

In specimen 36, a content of silicon (Si) was high, so that a fractionof fresh martensite was excessive. As a result, a balance (B_(TE)) oftensile strength and elongation was less than 3.0*10⁶, and a balance(B_(TH)) of tensile strength and a hole expansion ratio was less than6.0*10⁶, in specimen 36.

In specimen 37, a content of Aluminum (Al) was high, so that a fractionof fresh martensite was excessive. As a result, a balance (B_(TE)) oftensile strength and elongation was less than 3.0*10⁶, and a balance(B_(TH)) of tensile strength and a hole expansion ratio was less than6.0*10⁶, in specimen 37.

In specimen 38, a content of manganese (Mn) was low, so that pearlitewas generated, and a fraction of retained austenite was insufficient. Asa result, a balance (B_(TE)) of tensile strength and elongation was lessthan 3.0*10⁶, in specimen 38.

In specimen 39, a content of manganese (Mn) was high, so that a fractionof fresh martensite was excessive. As a result, a balance (B_(TE)) oftensile strength and elongation was less than 3.0*10⁶, and a balance(B_(TH)) of tensile strength and a hole expansion ratio was less than6.0*10⁶, in specimen 39.

In specimen 40, a content of chromium (Cr) was high, so that a fractionof fresh martensite was excessive. As a result, a balance (B_(TE)) oftensile strength and elongation was less than 3.0*10⁶, and a balance(B_(TH)) of tensile strength and a hole expansion ratio was less than6.0*10⁶, in specimen 40.

In specimen 41, a content of molybdenum (Mo) was high, so that afraction of fresh martensite was excessive. As a result, a balance(B_(TE)) of tensile strength and elongation was less than 3.0*10⁶, and abalance (B_(TH)) of tensile strength and a hole expansion ratio was lessthan 6.0*10⁶, in specimen 41.

In specimen 42, a content of boron (B) was low, so that boron (B) wasnot concentrated into tempered martensite. As a result,[B]_(FM)/[B]_(TM) was more than 0.55, and a yield ratio evaluation index(I_(YR)) was more than 0.42, in specimen 42.

In specimen 43, a content of boron (B) was high, so that boron (B) wasexcessively concentrated into tempered martensite. As a result,[B]_(FM)/[B]_(TM) was less than 0.03, and a yield ratio evaluation index(I_(YR)) was less than 0.15, in specimen 43.

While the present disclosure has been described in detail throughexemplary embodiment, other types of exemplary embodiments are alsopossible. Therefore, the technical spirit and scope of the claims setforth below are not limited to exemplary embodiments.

1. A high strength steel sheet having excellent workability, comprising:by wt %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to0.005%, a balance of Fe, and unavoidable impurities; and asmicrostructures, bainite, tempered martensite, fresh martensite,retained austenite and unavoidable structures, wherein the high strengthsteel sheet satisfies the following relational expression 1:0.03≤[B] _(FM) /[B] _(TM)≤0.55  [Relational Expression 1] where [B]_(FM)is a content (wt %) of Boron (B) contained in the fresh martensite, and[B]_(TM) is a content (wt %) of Boron (B) contained in the temperedmartensite.
 2. The high strength steel sheet of claim 1, furthercomprising: by wt %, one or more of the following (1) to (8): (1) one ormore of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5%; (2) one or moreof Cr: 0 to 3.0% and Mo: 0 to 3.0%; (3) one or more of Cu: 0 to 4.0% andNi: 0 to 4.0%; (4) one or more of Ca: 0 to 0.05%, REM: 0 to 0.05%excluding Y, and Mg: 0 to 0.05%; (5) one or more of W: 0 to 0.5% and Zr:0 to 0.5%; (6) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%; (7) oneor more of Y: 0 to 0.2% and Hf: 0 to 0.2%; and (8) Co: 0 to 1.5%.
 3. Thehigh strength steel sheet of claim 1, wherein the microstructures of thesteel sheets include, by volume fraction, 10 to 30% of bainite, 50 to70% of tempered martensite, 10 to 30% of fresh martensite, 2 to 10% ofretained austenite, and 5% or less (including 0%) of ferrite.
 4. Thehigh strength steel sheet of claim 1, wherein a balance (B_(TE)) oftensile strength and elongation expressed by the following relationalexpression 2 satisfies 3.0*10⁶ to 6.2*10⁶ (Mpa²%^(1/2)), a balance(B_(TH)) of tensile strength and a hole expansion ratio expressed by thefollowing relational expression 3 satisfies 6.0*10⁶ to 11.5*10⁶(Mpa²%^(1/2)), and a yield ratio evaluation index (I_(YR)) expressed bythe following relational expression 4 satisfies 0.15 to 0.42:B _(TE)=[Tensile Strength (TS, MPa)]²*[Elongation (El,%)]^(1/2);  [Relational Expression 2]B _(TH)=[Tensile Strength (TS, MPa)]²*[Hole Expansion Ratio (HER,%)]^(1/2); and  [Relational Expression 3]I _(YR)=1−[Yield Ratio (YR)].  [Relational Expression 4]
 5. A method formanufacturing a high strength steel sheet having excellent workability,the method comprising: providing a cold-rolled steel sheet including, bywt %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to0.005%, a balance of Fe, and unavoidable impurities; heating (primarilyheating) the cold-rolled steel sheet to a temperature of 700° C. at anaverage heating rate of 5° C./s or more, heating (secondarily heating)the primarily heated steel sheet to a temperature within a range of Ac3to 920° C. at an average heating rate of 5° C./s or less, and thenmaintaining (primarily maintaining) the secondarily heated steel sheetfor 50 to 1200 seconds; cooling (primarily cooling) the primarilymaintained steel sheet to a temperature within a range of 200 to 400° C.at an average cooling rate of 1° C./s or more; heating (tertiarilyheating) the primarily cooled steel sheet to a temperature within arange of 350 to 550° C. at an average heating rate of 5° C./s or more,and then maintaining (secondarily maintaining) the tertiarily heatedsteel sheet for 50 seconds or more; and cooling (secondarily cooling)the secondarily maintained steel sheet to room temperature at an averagecooling rate of 1° C./s or more.
 6. The method of claim 5, wherein thesteel slab further includes one or more of the following (1) to (8): (1)one or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5%; (2) oneor more of Cr: 0 to 3.0% and Mo: 0 to 3.0%; (3) one or more of Cu: 0 to4.0% and Ni: 0 to 4.0%; (4) one or more of Ca: 0 to 0.05%, REM: 0 to0.05% excluding Y, and Mg: 0 to 0.05%; (5) one or more of W: 0 to 0.5%and Zr: 0 to 0.5%; (6) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%;(7) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%; and (8) Co: 0 to1.5%.
 7. The method of claim 5, wherein the cold-rolled steel sheet isprovided by: heating steel slab to 1000 to 1350° C.; performingfinishing hot rolling at a temperature within a range of 800 to 1000°C.; coiling the hot-rolled steel sheet at a temperature within a rangeof 350 to 600° C.; pickling the coiled steel sheet; and cold rolling thepickled steel sheet at a reduction ratio of 30 to 90%.